In the related art, a plasma processing apparatus using a radial line slot antenna has been known as a plasma processing apparatus that performs a predetermined plasma processing on a workpiece such as, for example, a semiconductor wafer. The radial line slot antenna is disposed on a ceiling opening a processing container in a state where a slow wave plate is disposed above a slot plate having a plurality of slots and is connected to a coaxial waveguide at the central portion thereof. With this configuration, microwaves generated by a microwave generator are radially transmitted in a radial direction by the slow wave plate via the coaxial waveguide, generate circular polarized waves by the slot plate, and then are radiated from the slot plate into the processing container. High density plasma having a low electron temperature may be generated in the processing container by the microwaves under a low pressure and the plasma processing such as, for example, a film forming processing or an etching processing is performed by the generated plasma.
In the plasma processing apparatus described above, for example, the coaxial waveguide 100 is configured by an inner conductor 101 and an outer conductor 102, as illustrated in FIG. 7. The lower end portion 101a of the inner conductor 101 is formed in a conical shape and has a tapered shape of which a diameter increases downwardly. Further, the slow wave plate 110 has, for example, a protrusion 110a which upwardly protrudes therein. The protrusion 110a is formed to fill a gap between the inner conductor 101 and the outer conductor 102 and suppresses an axial deviation in a radial direction of the coaxial waveguide 100.
However, since the protrusion 110a in the slow wave plate 110 is hard to process, minute air layers 111 are generated between the protrusion 110a and the inner conductor 101 or between the protrusion 110a and the outer conductor 102. Thus, the transmission path of the microwaves becomes a complicated space. Further, the air layers 111 may become different from each other depending on a machine difference between apparatuses. Then, the microwaves are not appropriately transmitted from the coaxial waveguide 100 to the slow wave plate 110, and as a result, the circumferential radiation of the microwaves in the slow wave plate 110 becomes non-uniform.
From this point, for example, Japanese Patent Laid-Open Publication No. 2011-077228 discloses a slow wave plate 120 which does not have the protrusion 110a, as illustrated in FIG. 8. The slow wave plate 120 is configured by a small diameter circular flat plate member disposed therein and a large diameter circular flat plate member which encloses the small diameter member. FIG. 8 illustrates the small diameter member of the slow wave plate 120. The inner surface 120a of the slow wave plate 120 is located more inside than the inner surface 102a of the outer conductor 102 of the coaxial waveguide 100 in the radial direction. In other words, the slow wave plate 120 is provided to protrude toward the inner conductor 101 more than the outer conductor 102.